1. Field of the Invention
This invention relates to a master cylinder for sending oil under pressure to a driven unit such as a wheel cylinder.
2. Description of Related Art
FIG. 9 partially shows a conventional master cylinder.
This master cylinder has a piston guide 2 disposed in a cylinder housing. The piston guide 2 has a through hole 3 drilled therein so that a passage 4 which is in communication with an oil reservoir (not shown) opens to a pressure chamber 5 via the through hole 3 in the inoperative state. A piston 6 has a through hole 7 formed in its cylindrical wall. As shown in FIG. 9, the pressure chamber 5 communicates with the oil reservoir (not shown) through the through hole 7, a clearance portion 38 between the piston 6 and the piston guide 2, and the through hole 3 of piston guide 2. Therefore, if the oil in the pressure chamber 5 falls short, oil is supplied from the oil reservoir to the pressure chamber 5.
In this master cylinder, an annular seal 8 is attached to the side surface of piston guide 2 at the side of pressure chamber 5 (left side surface in FIG. 9). When the piston 6 is pushed into the pressure chamber 5 and the through hole 7 of piston 6 passes through the annular seal 8, the communication between the through hole 7 and the through hole 3 of piston guide 3 is cut off. When the piston 6 is pushed further into the pressure chamber 5, the oil in the pressure chamber 5 is pressurized and fed to a driven unit such as a wheel cylinder (not shown).
When the piston 6 returns (moves to the right in FIG. 9), the pressure in the pressure chamber 5 becomes negative, so that the external portion 8a of annual seal 8 disengages from the internal surface of a sleeve 9 (the internal surface of cylinder) as shown by the dash-and-dot line in FIG. 10. As a result, a narrow passage 10 is formed between them. Consequently, an oil path is formed by the passage 10, a passage 11 formed between the internal surface 9a of sleeve 9 and piston guide 2, the through hole 3 in the piston guide 2 and the passage 4, through which oil is supplied from the oil reservoir (not shown) to the pressure chamber 5. Thus, the master cylinder is so constructed that the action of the driven unit is not hindered by the negative pressure in the pressure chamber 5.
In the master cylinder of such a type, it is desirable to make the passage (gap) 11 as large as possible in order to ensure good pumping properties by smoothly supplying oil to the pressure chamber, for example, in the pumping brake operation in which a brake pedal is depressed repeatedly. However, if the passage 11 is made large, there is a risk of the edge portion 8b of annular seal 8 (refer to FIG. 10) being deformed elastically and intruding into the gap between the internal surface 9a of sleeve 9 and the external surface 2a of piston guide 2 when the piston moves toward the pressure chamber 5.
If this occurs, the edge portion 8b of annular seal 8 is damaged, resulting in decreased durability. Also, the abrasion powder enters the passage 11, which hinders the formation of passage during the brake pumping operation, leading to the disturbance of a smooth brake pumping operation.
One possible solution to these problems is that a communicating passage A is formed in the piston guide 2 as shown by the dash-and-dot line in FIG. 10. The communicating passage A is in communication with the passage 3 and opens to the side surface 2b of piston guide 2 so that the communication passage A serves as an oil passage to ensure large oil passage during the pumping operation. In this configuration, when the annular seal 8 is urged against the side surface 2b of piston guide 2, a part of the abutting surface of annular seal 8 is deformed elastically, and may intrude into the communicating passage A in the piston guide 2. When the communicating passage A is formed in the piston guide 2 as described above, therefore, an annular spacer B may be interposed between the piston guide 2 and the annular seal 8 as shown by the dash-and-dot line in FIG. 10, which, however, presents a difficulty in easy assembling and positioning of the annular spacer B. For example, if the annular spacer B is so designed that it is assembled with the piston guide 2 being used as a guide on the internal surface, there is a problem of difficult assembling of the annular spacer B and its associated parts.